1. Field of the Invention
The present invention relates to hydraulic systems and, more specifically, to a hydraulic system that is configured to store and reuse energy.
2. Description of the Related Art
Many elevators are actuated by hydraulic systems. Typical hydraulically actuated elevators include a cab for transporting people and things. A hydraulic cylinder and piston move the cab up and down. To the cab is moved up by pumping hydraulic fluid from a reservoir into the cylinder and the cab is moved down by allowing hydraulic fluid to flow from the cylinder back into the reservoir.
Controlling the speed of the elevator can be challenging as people generally do not like abrupt starts and stops as the elevator ascends and descends. Several shortfalls exist in the speed-control systems currently employed in hydraulic systems. Most currently used speed-control systems employ two general approaches, each with its own advantages and disadvantages: (1) speed control via fluid throttling (typically implemented in some type of load sensing circuit) to regulate flow into the actuator; or (2) speed control employing an electric motor to control flow output of the pump. Throttling is easily and inexpensively implemented in a hydraulic system; however, energy dissipates throughout the motion of the actuator, thereby losing much of the energy employed to move the actuator. This can be a major source of inefficiency and tends to produce higher fluid requirements due to fluid heating. Speed control with an electric motor can be implemented with either a single speed electric drive operating a variable-displacement pump (displacement control), or with a fixed-displacement pump using a variable speed electric drive. Either approach can yield energy savings in comparison to throttling, and in some applications connected to a power grid (such as an elevator), the energy can be regenerated by converting some of the mechanical energy expended by the resistance of motion into electrical energy for storage in the power grid. However, this regeneration employs a relatively inefficient mechanical-to-electrical energy conversion and requires a mechanism for the electric utility company to reimburse the operator for returned electrical energy.
In a hydraulic elevator, motion can be adequately controlled through both throttling and displacement control. The most recent work on hydraulic elevators suggests regenerating energy through an electric motor converting mechanical to electrical energy. While an improvement over throttling, this energy conversion results in unacceptable inefficiencies.
In recent years hydraulic elevators have lost market share to other technologies. In 1986, hydraulic elevators comprised approximately 70% of all elevators sold in the US. By 1995, the market share began decreasing due to the introduction of machine room-less (MRL) traction elevators, recently reaching 40%. Additionally, traction elevators have long boasted a higher efficiency than hydraulic elevators as a result of the counterweight used to reduce the amount of net weight the motor experiences. Even on counterweighted hydraulic elevators, the throttling valve used during motion dissipates a significant amount of energy and negatively impacts the hydraulic elevator's efficiency. The recent loss in competitiveness, together with the assessment that 3-8% of electricity consumption of a building is attributed to elevator operation, provides sufficient motivation to improve current speed control and energy regeneration techniques in hydraulic elevators.
Therefore, there is a need for a hydraulic system that can be employed to actuate an elevator that can recapture, store and reuse energy from the elevator.